Improvements in construction materials, construction methods, and more stringent local and state building codes have contributed to improved energy efficiency of new and remodeled insulated wall structures for homes and buildings.
The conventional approach to fabricating a highly energy-efficient wall is to erect a wall frame supporting multiple layers of insulation placed between interior and exterior layers of the wall. One or more breathable “house-wrap” styled layers is secured (e.g., stapled) to an exterior sheathing surface to prevent bulk water from wetting the insulation and thus reducing its insulative value (R-value), as well as wetting the sheathing and framing causing mold and rot. Typically, a low permeance (<0.1 perm polyethylene membrane) is attached to the warm-in-winter side of the framing members. Continuing experience shows that the combined effect of dry sheathing and a warm-side vapor retarder results in walls that have a tendency to retain moisture, which can undesirably lead to mold growth within the wall, degradation of the wall, insects, and/or other moisture-related problems. These conventional insulated wall structures also reduce heat loss through the wall by reducing drafts (infiltration) that remove heat from the home/building. However, since these conventional insulated wall structures are so tightly constructed/sealed, any water that is trapped in the wall (e.g., due to a breach or damage to the structure or to condensation build-up) tends to remain inside the wall. Moisture that is trapped inside a wall reduces the performance of the insulation and has the potential to feed the growth of mold and/or bacteria.
Moisture trapped inside of the walls includes moisture vapor and bulk water, such as condensation. Condensation can form inside a wall due to temperature differences across the insulated walls. For example, during typical northern cold winter months, the air outside of an insulated wall is cold and dry, and the air inside of the wall is relatively warm and humid. Thus, a natural humidity gradient is formed that drives moisture vapor in the air inside the wall toward the exterior of the wall. Large gradients between outside and inside air temperature and humidity can lead to a significant accumulation of moisture condensation within the insulated wall.
The opposite conditions occur during the summer months, when the air outside the structure is warm and humid, and the air inside the structure is conditioned to be cooler and dryer. Thus, during summer months a natural humidity gradient exists to drive warm humid air toward an interior of the insulated wall, which can analogously lead to a significant accumulation of moisture condensation within the insulated wall.
In some cases moisture accumulation in the insulated wall arises from wind driven water that enters the wall along a window or door seam. This form of moisture ingress can, for example, be the result of poor workmanship or from a deterioration of flashing or sealants around the window/door. In any regard, once the wall accumulates moisture it is difficult to dry the wall to a level that will not support the growth of mold and/or bacteria.
Owners, manufacturers, and remodelers of wall structures desire walls that are energy efficient, durable, and compatible with accepted construction practices.